This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-291909, filed Sep. 26, 2000, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a light-emitting diode (LED) driving circuit and an optical transmission module using the circuit and, more particularly, to a high-speed optical transmission module or an optical transmission module used in an optical transceiver.
2. Description of the Related Art
A conventional light-emitting diode (LED) driving circuit for obtaining a high-speed optical output waveform uses a circuit for shortening a fall time and is designed to perform peaking at a leading edge portion, as disclosed in Jpn. Pat. Appln. KOKAI Publication No. 7-74788 xe2x80x9cHigh-Speed Optical Transmission Circuitxe2x80x9d.
FIG. 6 shows a conventional light-emitting diode driving circuit. As shown in FIG. 6, the inputs of a first current switch circuit 33, buffer amplifier 34, and inverter 35 are connected to an input terminal 32. One terminal of a first capacitor 36 is connected to the output of the buffer amplifier 34. A second current switch circuit 38 is connected to the other terminal of the first capacitor 36. One terminal of a second capacitor 37 is connected to the output of the inverter 35. A third current switch circuit 39 is connected to the other terminal of the second capacitor 37. A current mirror circuit 40 formed by a PNP transistor is connected to the third current switch circuit 39. An LED 1 is connected to the current mirror circuit 40. The first and second current switch circuits 33 and 38 are connected to the cathode of the LED 1. Note that a power supply potential Vcc is applied to the anode of the LED 1.
According to the above conventional driving circuit, a driving current to the LED 1 is turned on/off by the first current switch circuit 33, and a digital signal passing through the buffer amplifier 34 is differentiated by the first capacitor 36. The leading edge of this differentiated digital signal is converted into a current signal by a comparator+second current switch circuit 38 to be supplied to the LED 1. The digital signal inverted by the inverter 35 is also differentiated by the second capacitor 37. The trailing edge of this differentiated digital signal is converted into a pulse current signal by a comparator+third current switch circuit 39 to be supplied to the current mirror circuit 40. An output signal from this current mirror circuit 40 is then supplied to the cathode of the LED 1.
According to this arrangement, at the leading edge of the above output signal, the LED 1 is overdriven, whereas at the trailing edge of the output signal, the charge stored in the LED 1 is discharged. This makes it possible to drive the LED 1 at high speed.
The following three problems, however, arise in the above conventional driving circuit.
First, since a lateral type PNP transistor having a low cutoff frequency is often used as a PNP transistor that can be used in an IC, a high-speed pulse current cannot be supplied. Although a vertical type PNP transistor having a relatively high cutoff frequency may be formed, a high-speed pulse current cannot be supplied to this transistor as compared with an NPN transistor. In addition, since the parasitic capacitance between the collector and the substrate is large, a sufficiently high operation speed cannot be attained when the transistor is driven at 100 Mpbs or more. It is therefore difficult to implement sufficiently high-speed operation.
Second, since a pulse current is generated by a differential signal obtained by a capacitor, it is difficult to design the pulse width of a pulse current supplied to the LED to have a desired value. For this reason, it is difficult to match the pulse width of the pulse current generated by a differential signal to the pulse width of a pulse current for discharging the charge stored in the LED. It is therefore difficult to obtain a desired optical output waveform.
Third, the LED has a transition time capacitance component of about 100 to 200 pF as well as a general junction capacitance. This transition time capacitance component increases in proportion to a current that flows in the LED. For this reason, when a current is supplied to the LED, the current is used first to charge the LED with a transition time capacitance component. This produces a delay time of about 10 to 20 ns before the optical output reaches a desired value. At the trailing edge of the output signal, since the current ceases to flow with a slight drop in voltage, the delay time is shorter than that at the leading edge. Consequently, the pulse width of light becomes smaller than that of an input. That is, the conventional circuit structure is not suited to high-speed transmission.
According to the first aspect of the present invention, there is provided a light-emitting diode driving circuit comprising a light-emitting diode, a first current switch circuit connected in series with the light-emitting diode, the first current switch circuit turning on/off a current in accordance with an external input signal input from an input terminal, a pulse current generating circuit connected in parallel with the first current switch circuit, the pulse current generating circuit supplying a pulse current including a pulse width smaller than a pulse width of the external input signal and shaping a leading edge portion of an optical waveform output from the light-emitting diode into a desired optical waveform, and a discharge circuit connected in parallel with the light-emitting diode, the discharge circuit quickly discharging charge stored in the light-emitting diode when the current to the first current switch circuit is turned off.
According to the second aspect of the present invention, there is provided an optical transmission module comprising a sub-module substrate or a lead frame, a light-emitting diode mounted on the sub-module substrate or the lead frame, an IC mounted on the sub-module substrate or the lead frame and including a light-emitting diode driving circuit for driving the light-emitting diode, an optical connector optically coupled to the light-emitting diode, a lead electrically coupled to the IC or the light-emitting diode, and a package for housing the sub-module substrate or the lead frame, the optical connector, and the lead. The light-emitting diode driving circuit comprises a first current switch circuit connected in series with the light-emitting diode, the first current switch circuit turning on/off a current in accordance with an external input signal input from an input terminal, a pulse current generating circuit connected in parallel with the first current switch circuit, the pulse current generating circuit supplying a pulse current including a pulse width smaller than a pulse width of the external input signal and shaping a leading edge portion of an optical waveform output from the light-emitting diode into a desired optical waveform, and a discharge circuit connected in parallel with the light-emitting diode, the discharge circuit quickly discharging charge stored in the light-emitting diode when the current to the first current switch circuit is turned off.